Travelling south on Legacy. You get a red light. Light turns greens but there are already cars in the intersection from the second red light. They cannot go because they don't get an immediate green arrow but only green light. They get green arrow but then only 4 cars get through. Then the cars waiting are now stuck at the second part of the intersection.

Second issue is safety. You get a green light and expect to go but cars immediately slam on their breaks and sometimes stop in the intersection.

Because of the drainage ditch that runs between westbound and eastbound Main, the intersection functions as two separate intersections similar to the intersections you drive through along Dallas Parkway or any other freeway interchange in the Dallas area that has frontage roads. At this location, the separation between both directions of Main Street is less than the distance between the two service roads which makes it more difficult to operate efficiently. There are changes that can be made to increase efficiency, but it would be at the expense of safety. In the 12 month period from October 1, 2007 through September 30, 2008 there were 16 red light running crashes. Overall during that 12 month period, there were an average of 2.6 crashes per month. In October 2008, we made changes to the physical signal itself as well as changed the signal timing. In the 18 month period from October 2008 through March 2010, there have been 6 red light running crashes at this intersection. The overall crash rate during that 18 month time frame is now 0.94 crash per month. This is a significant reduction that is due to the changes to the traffic signal and signal timing.

The intersection is currently under review and is having new signal timing developed as part of a larger effort to retime 58 of our 74 traffic signals. This new timing will be ready to implement by late August after the school year resumes. The consultant has been provided the crash information for this location and is taking that into account when developing the new signal timing.

In addition to the signal timing project, we have a consultant preparing preliminary engineering plans for intersection reconstruction. They are finishing up an operational analysis of the various geometric changes that can be made to improve the safety and efficiency of the intersection. Once the analysis and preliminary plans are completed, we can then get the project into the Capital Improvements Plans (CIP) so it can be programmed for funding.

We are constantly monitoring high capacity intersections, such as this one, during the peak periods of the day remotely at our Emergency Operations Center using traffic cameras. During the 5 PM peak hour, we recently observed an average of 5-6 vehicles, per lane, getting through this intersection southbound. That may occasionally leave a vehicle that is unable to make it through this light southbound. Our primary concern with this intersection during the 5 PM hour is the 2,325 vehicles heading westbound on Main Street. The 340 southbound vehicles in the 5 PM hour make up a much smaller portion of the total traffic at this intersection (4,716 total vehicles) and the amount of green time is distributed around the intersection based on the volumes of each approach. Currently, the southbound approach is getting the appropriate amount of green time based on the volumes.

Referring back to the July 6th post above from Brian Moen, this intersection is currently under review to be improved. It is having new signal timing developed as part of a larger citywide retiming effort and there is a consultant preparing preliminary engineering plans for reconstructing this intersection. They are finishing up an operational analysis of the various options for geometric changes that can be made to improve the safety and efficiency of the intersection.

This intersection was signalized over 5 years ago. According to our accident analysis of this intersection, the improved signal sequencing and the mast arm stabilized signal heads on the farside of the intersection (going north and south) significantly reduced the number of accidents once it was implemented in October 2008.

The signal timing at this location was changed on October 6th. All of the new timings have been implemented and the follow up travel time runs are currently underway to assess the changes. Those will be completed by early December.

The time for southbound Legacy did increase during the time in question in comparison to the previous timing. The arrow does come on each cycle and has done so each weekday during the time in question since October 6th. During Monday through Friday, the arrow is programmed to come on regardless of demand from 5:30am to 10pm. From 4pm to 7pm, the arrow can stay on for a minimum of 3 seconds and can be extended up to 14 seconds provided there are vehicles travelling over the detectors to extend the time. We will see if further changes can be made without causing a breakdown in the progression of traffic along Main Street. Even if small changes are made, there will still be individual cycles where the demand could exceed the arrow time provided and motorists will either need to turn on the green ball or wait until the next cycle. To guarantee that every southbound left-turn vehicle will make it each cycle will increase the overall delay and fuel consumption for the entire intersection at the benefit of a low volume movement. The timing during this three hour period is based on the peak hour which occurs from 5:30pm to 6:30pm where the traffic volume on Main Street is three times the traffic volume on Legacy.

Brian Moen Assistant Director of Engineering

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